Control of fading in radio communication systems



F eb. 2, 1943..

Filed oct. es,l 1940 l s sheetsp'shget 1 2 SN s3 INVENTOR A. L. RE N ATZI'ORNEY A. L. GREEN 2,310,196A

Filed 001:. 8, 1940 3- Sheets-Sheet 2 Feb. 2, 1943.

CONTROL `OF FADING IN RADIO COMMUNICATION SYSTMS Feb. 2, 1943. M L, GREEN 2,310,196 CONTROL OF FADING IN RADIO COMMUNICATION SYSTEMS 4 Filed oct. s. 1940 s sheets-sheet s y Figa@ AMPLI/75K PHONES Riem-1ER l ATTORNEY Patented Feb. 2, 1943 CONTROL OF FADING IN RADIO COM- MUNICATION SYSTEMS Alfred Leonard Green, Sydney, New South Wales, Australia, assigner to Amalgamated Wireless (Australasia) Wales, Australia, Wales Limited,

a corporation of New South Sydney, New South Application October 8, 1940, `Serial No. 360,231 In Australia October 26, 1939 6 Claims.

This invention relates to improvements in the control of naturally occurring intensity variations in telecommunication systems, for example, selective fading in radio communication.

In long distance radio communication it is observed that both the intensity and the fidelity of the reproduced intelligence vary rapidly with respect to time in an undesirable manner. `In order to overcome such defects it is known to transmit carrier-wave signals simultaneously on adjacent frequencies and, in such controlled systems of telecommunication, it has been observed that the combined signals are less subject to undesirable fading effects than is a carrier-wave signal transmitted on a single frequency. In the simpler type of transmission in which only one carrier-wave frequency is employed, it is observed that the energy at carrier-wave frequency arriving at the receiver undergoes violent fluctua-A tions in intensity and that during periods of low carrier intensity the reproduced intelligence is marred by distortion. On the other hand, the transmission of duplicate carriers on adjacent frequencies avoids this type of undesirable distortion since there is then available, either at one carrier frequency or another, carrier energy which is utilized in combination with the transmitted high frequency modulation side-bands to reproduce the desired intelligence.

Nevertheless, even in such improved types of telecommunication systems in which duplicate carriers are employed it is observed that the iidellty of the reproduced intelligence is subject to deterioration and that, by way of example, there is a noticeable reduction in the reproduced intensity of some tones.

Generally it is observed that deterioration in fidelity is of a relatively stable nature when the telecommunication circuit is at a frequency of the order of one megacycle, but that transmission at higher frequencies is subject to selective fading whereby the reproduced intensity of a specied tone in the intelligence is subject to relatively rapid variations with respect to time.

The principal object of the present invention is, therefore, to provide an improved method for the transmission and/or reception of modulated high frequency (H. F.) oscillations adapted for carrying out communication over long distances in which means are provided for reducing to a minimum controllable distortion of the type associated with carrier fading, and for substantially eliminating the deterioration in fidelity of the reproduced intelligence which is brought about by selective fading of the modulation side-bands.

The above objects are achieved in the improved radio communication system constituting this invention, which comprises means for transmitting and receiving one or more duplications of the desired intelligence on a common H. F. carrier, characterized in this that each of saidduplications is contained in separate modulation side-bands, that means are provided for main taining a frequency separation between the frequency of a signal in one of said side-bands and the frequency of a. second signal in another of said side-bands, said second signal corresponding with the same low frequency (L. F.) of the said intelligence as the first mentioned signal, and that the value of said frequency separation is independent of the frequency of said intelligence.

The method of carrying the invention into effect may best be understood by reference to the following description and to the attached diagrams in which like parts are distinguished by like reference numbers.

Figure 1 indicates a simple form of simultaneous transmission of H. F. modulation sidebands on adjacent frequencies;

Fig. 2 indicates an arrangement of transmitting apparatus for the transmission of a carrier wave and H. F. modulation sidebands in accordance withthe invention;

Fig. 3 indicates an arrangement of receiving apparatus for combining the transmissions and for reproducing the desired intelligence;

Fig. 4 indicates another portion of the receiving apparatus, comprising alternative means for combining the received signals; and

Fig. 5 indicates a portion of the receiving apparatus embodying noise-suppression means for use in radio telephone speech systems.

The system of simultaneous transmissions indicated by Fig. 1 provides, by way of a nonlimiting example, a carrier wave l at a frequency of 10 megacyclesy a H. F. modulation sideband 2 occupying a band of frequencies between 10,000,250 cycles and 10,002,500 cycles, and a duplicate H. F. modulation sideband 3 occupying" a band of frequencies between 10,003,250 cycles and 10,005,500 cycles.

The system of transmission is such that the limiting frequencies 10,000,250 and 10,002,500 cycles in modulation sideband 2 correspond respectively with the limiting frequencies 250 and 2,500 cycles in the band of low frequencies of the intelligence to be transmitted which, in the present example, may be speech. Similarly the signals in modulation sideband 3 having the limiting frequencies of 10,003,250 cycles and 10,005,500 4 cycles correspond respectively with those portions of the speech intelligence to be transmitted having respectively the limiting frequencies 250 and 2,500 cycles. Thus a specified portion of the intelligence to be transmitted, for example, that portion having a frequency of 400 cycles, is transmitted in modulation sideband 2 at a frequency of 10,000,400 cycles and simultaneously in modulation sideband 3 at a frequency of 10,003,400 cycles. Similarly the system provides for the duplicate transmission of any other portion of the intelligence and it should particularly be noted that the frequency separation between a signal in one sideband and a duplicate correspending signal in the other sideband is, in the present example, always 3 kilocycles.

It is clear, however, that values of the frequency separation other than 3 kilocycles may be used with advantage. It is known that selective fading is due, at least in part, to destructive phase interference between the two main sky waves arriving at the receiver as components of a signal in a H. F. modulation sideband. Denoting the equivalent path-difference between two such sky waves as D kilometres, it may then be shown that control of selective fading is achieved according to the invention when the following relation is satised viz.:

where f2 and f3 are the frequencies of corresponding signals in modulation sidebands 2 and 3 respectively, n is an integral number and c is the velocity of wave propagation. In the example illustrated in Fig. 1 the value of D is 250 kilometers and the integer 2 is chosen for n so that the required value of the frequency separation f3-f2 is 3,000 cycles.

Under other conditions of wave propagation the value of the sky wave path difference may, for example, be 300 kilometers and it may be convenient to choose the integer 4 for n. In this case the desired value of the frequency separation fS-JZ is 4,500 cycles so that assuming, as in the example illustrated in Fig. 1, that the carrier frequency is 10 megacycles and that one H. F. modulation sideband extends from 10,000,250 cycles to 10,002,500 cycles, it then follows that control of selective fading is achieved according to the invention when the duplicate modulation sideband occupies a band of frequencies between 10,004,750 cycles and 10,007,000 cycles. In this example a specified tone in the L. F. intelligence, for example, 400 cycles, is transmitted in one modulation sideband at a frequency of 10,000,400 cycles and simultaneously in the duplicate modulation sideband at a frequency of 10,004,900 cycles.

Fig. 2 indicates a preferred arrangement of transmitting apparatus suitable for the production of the signals indicated by Fig. 1. The source of L. F. intelligence 4 may, for example, be incoming speech which is passed through the first band pass filter 5 having the characteristic that signals at frequencies lower than 250 cycles and higher than 2,500 cycles 'are greatly attenuated. A portion of the output from the rst band pass filter 5 is supplied to a first amplifier I whose amplified output is conducted to the channel combining amplifier II. A portion of the output from the first band pass filter is also supplied to a first modulator 6 to which is also connected a first oscillator I having a frequency'of 3 k; c.' The output from the first modulator 6 is applied to the second band pass lter 8 having the characteristic that all signals having frequencies higher than 5,500 cycles and lower than 3,250 cycles are greatly attenuated. Thus the second band pass filter 8 selects from the output of the first modulator 6 the upper sideband component having frequencies between 3,250 and 5,500 cycles and rejects both the first carrier at a frequency of 3 k. c., and also the lower sideband component having frequencies between 2,750 and 500 cycles. The selected output from second band pass filter 8 is then passed through the second amplifier 9 to the channel combining amplifier II.

The output from the channel combining amplifier Il, therefore, contains intelligence in the band of frequencies from 250 to 2,500 cycles and also a duplicate upper sideband component having frequencies lying between 3,250 and 5,500 cycles. For each and every tone in the original speech frequencies there is also present a corresponding signal whose frequency is equal to that of the specified tone plus a constant frequency difference of 3 k. c.

The remainder of the transmitting apparatus then consists essentially of convenient means for converting the combined outputs of channel combining amplifier I I to H. F. modulation sidebands and for transmitting said sidebands together with a carrier wave. In general this result; is conveniently achieved by a process of double modulation in which firstly a second oscillator I3 at a medium frequency of 100 k. c. is modulated by the speech and duplicate speech derived from channel combining amplifier I I.

'I'he output from the channel combining amplifier I I is, therefore, conducted to the second modulator I2 to which there is also applied the output from the second oscillator I3 operating at a frequency of k. c. The output from the second modulator I2 consists of a second carrier at a frequency of 100 k. c., two associated upper sidebands having frequency bands respectively of 100,250 cycles to 102,500 cycles and 103,250 cycles to 105,000 cycles, and in addition two corresponding lower sidebands occupying frequencies in the range from 94,500 cycles to 99,750 cycles. The third band pass filter I4, to which the output of second modulator I2 is applied, rejects all bands having frequencies lower than 100 k. c. and passes signals having frequencies between 100 k. c. and 105,500 cycles, that is t0 say, passes the second carrier at 100 k. c. and the two associated upper sidebands. In some cases, however, it is convenient to arrange the frequency characteristic of third band pass filter I4 such that the second carrier at a frequency of 100 k. c. suffers attenuation and is thereby reduced in intensity with respect to the associated upper sidebands which occupy the frequency bands from 100,250 to 102,500 cycles and 103,250 to 105,500 cycles respectively. In such cases the carrier radiated from the aerial system 20 is transmitted as a socalled pilot carrier. Y

The selected second carrier and associated upper sidebands inthe output from third band pass filter I4 are conducted to a first tuned amplifier I5 and from thence to a third modulator I5 to which there is also applied the output from third oscillator II operating at a frequency of 9,900 k. c. The output from third modulator I6 contains a signal at a frequency of 9,900 k. c., other signals in the frequency range from 9,794,500 cycles to 9,800 k. c.; a third carrierat a frequency of 10 m. c., an associated upper sideband occupying the band of frequencies between 10,000,250 cycles and 10,002,500 cycles, and a duplicate upper sideband occupying the band of frequencies between 10,003,250 cycles and 10,005,500 cycles. The output from third modulator |6 is conducted to fourth band pass filter I8, which rejects all signals having frequencies lower than m. c., and in particular greatly attenuates the signals respectively at a frequency of 9,900 k. c. and within the band from 9,794,500 cycles to 9,800 k. c. The fourth band pass filter I8 passes, however, the desired third carrier at a frequency of 10 m. c., and the upper sidebands occupying the frequency range from 10,000,250 cycles to 10,005,500 cycles. The output from fourth band pass filter i8 is then amplified in second tuned amplifier I9 and from thence conducted to the aerial and earth radiating system which, therefore, transmits, as illustrated in Fig. 1, a carrier wave at a frequency of 10 m. c., an associated H. F. upper modulation sideband occupyingr a band of frequencies from 10,000,250 cycles to 10,002,500 cycles, and a duplicate H. F. upper modulation sideband occupying a band of frequencies between 10,003,250 cycles and 10,005,500 cycles.

Fig. 3 indicates a preferred arrangement of receiving apparatus for receiving the signals indicated by Fig. 1 and reproducing the intelligence transmitted by the apparatus indicated by Fig. 2. In Fig. 3 the aerial and earth receiving system 2| collects signal energy transmitted in accordance with this invention and applies said signal energy to radio-frequency amplifier 22 which is tuned to resonate at a frequency within the range 10 m. c. to 10,005,500 cycles. The amplified output from radio-frequency amplifier 22 is conducted to mixer -23 to which there is also applied through conducting lead 5| the output from mixer-oscllator 24 which operates at a frequency such as will produce the desired intermediate frequency, by way of example, at a frequency of 9,500 k. c. The intermediate-frequency (I. F.) amplifier 25 selects from the output lead 45 of the mixer 23 in the present example, signals within the band of frequencies from 500 k. c. to 505,300 cycles, that is to say, an I. F. carrier signal at a frequency of 500 k. c., an associated upper sideband signal occupying a band of frequencies between 500,250 cycles and 502,500 cycles, and a duplicate upper sideband signal occupying a band of frequencies between 503,250 cycles and 505,500 cycles.

The output from I. F. amplifier 25 is fed to detector 26. The output from detector 26, therefore, contains intelligence in the band of L. F. from 250 cycles to 2,500 cycles but in addition there are present signals in the frequency band from 3,250 cycles to 5,500 cycles. The output from detector 26 is conducted to channel splitting amplifier 29, a portion of whose output is passed through first low-pass filter 30 to channel combining amplier and another portion is passed throughhigh-pass filter 3| to modulator 32. The frequency characteristic of first low-pass filter 30 is such that it selects from the amplifier output of detector 26 intelligence in the band of L. F. from 250 cycles to 2,500 cycles. The frequency characteristic of high pass filter 3| is such that it selects from the amplified output of detector 26 signals in the frequency band from 3,250 cycles to 5,500 cycles. The last mentioned signals are applied to modulator 32 to which there is also supplied energy at a frequency of 3 k. c. from first oscillator 33. The output from modulator 32 is passed through second low pass filter 34, which may have a similar frequency characteristic 'to that of first low pass filter 30, to channel combining ampliner 35 whose output is fed to telephones or line output 36.

Considering for the moment only the reception of carrier wave I at a frequency of 10 m. c. and H. F. modulation sideband 2 occupying a band of frequencies between 10,000,250 cycles and 10,002,500 cycles, it will be appreciated that components 2|, 22, 23, 24, 25, 20, 20, 30, 35 and 36 in the receiver comprise a superheterodyne receiver which is capable of reproducing the original intelligence derived from source of Fig. 2.

Considering, by way of example, a specified portion of the original speech intelligence in source comprising a tone at a frequency of 400 cycles, this tone is translated to a frequency of 10,000,400 cycles in the transmitting apparatus and emitted in high frequency modulation sideband 2. The conditions of propagation being such that at least a portion of the transmitted energy at 10,000,400 cycles is received in aerial system 2| together with some carrier energy at a frequency of 10 m. c., it follows that, after being combined in mixer 23 with energy from the mix- `er-oscillator 24 at a frequency of 9,500 k. c., there are present in I. F. amplifier 25 an incoming carrier at a frequency of 500 k. c. and an associated sideband tone at a frequency of 500,400 cycles. The signals applied to detector 26, therefore, comprise the sideband tone at a frequency of 500,400 cycles and a carrier at a frequency of 500 k. c. The output from detector 20 then contains the desired tone at a frequency of 400 cycles.

It' may happen under severe conditions of selective fading that the output from detector 26 at a. frequency of 400 cycles is markedly reduced on account of a reduction in intensity of the H. F. sideband tone at a frequency of 10,000,400 cycles and a corresponding reduction in intensity of the I. F. sideband tone at 500,400 cycles, thereby producing an undesirable reduction in delity of the reproduced tone at a frequency of 400 cycles.

According to this invention, however, the 400 cycle tone derived from source 4 is also transmitted in duplicate H. F. modulation sideband 3 at a frequency of 10,003,400 cycles at which H. F. the selective fading effects are different from those at a frequency of 10,000,400 cycles.

At such times, therefore, as reception at 10,000,400 cycles is with reduced intensity there is available compensating energy at 10,003,400 cycles which, in mixer 23, is translated to I. F. energy at 503,400 cycles. The input to the detector 26 then comprises carrier at 500 k. c., a sideband tone at 500,400 cycles with reduced intensity due to selective fading, and a duplicate compensating sideband tone at a frequency of 503,400 cycles. The output from the detector 26 includes compensating energy at 3,400 cycles which is passed through channel splitting amplifier 29 and through high pass filter 3| to modulator 32 in which the combination of compensating energy at 3,400 cycles with energy at 3 k. c. from first oscillator 33 produces, as desired, a compensating tone at a frequency of 400 cycles. This compensating tone is then passed through second low pass filter 34 to channel combining amplifier 35 and from thence to line output or telephone 36.

Although the foregoing description has been confined to a comparatively simple form of receiver suitable for carrying out the present invention, various modifications may be applied to the arrangement described in order to improve the eciency of its operation.

Some of the modifications and the methods of applying them to the receiver already described are as outlined in the further description in connection with Fig. 3.

The level of the signal energy applied to the input of the detector 26 may be kept substantially constant by automatically controlling the gain of the preceding radio frequency and I. F. stages in opposition to the Variations in carrier intensity. Automatic volume control (A. V. C.) potentials for this purpose may be derived in any well known manner, for example, from the output of the detector 26, or from the output of a separate detector 42 to which I. F. potentials are applied from a convenient point in the amplifier 25, either direct or through one or more additional stages of amplification. The A. V. C. potentials thus derived may be applied to the controlled valves either direct or by way of a direct current amplifier t'hrough a suitable time constant network.

In the present examples the desired A. V. C. potentials are obtained and utilized as shown in Fig. 3. Referring to that figure, it Will be seen that another portion of the output from I. F. amplier 25 is supplied through lead 41 to I. F. carrier pass filter 2l whose frequency characteristic is such that the carrier signal at a frequency of 500 k. c. is passed to I. F. carrier amplifier 31, while the associated upper sideband signal and the duplicate upper sideband signal, oc-

cupying frequency bands respectively of 500,250

cycles to 502,500 cycles, and 503,250 cycles to 505,500 cycles, are suppressed or greatly attenuated. A portion of the amplified carrier' signal output from I. F. carrier amplifier 3l is applied to A. V. C. diode 42 which generates A. V. C. bias for application through the timeconstant circuit 44 to radio-frequency amplifier 22 and to I. F. amplifier 25 by way of conducting lead 46.

Thus far the description of the receiver has been confined to the means for compensating for deterioration in fidelity of the reproduced signaldue to selective fading, i. e., fading ofv individual frequencies in the modulation sidebands as distinct' from distortion introduced into the output by fading of the carrier.

In a modification of the exemplary receiver described up to the present in connection with Fig. 3 however, facilities are provided for reducing to a minimum the distortion effects associated with a fading carrier and, for example, said facilities comprise, as indicatedV in Fig. 3, means for generating and injecting into detector 26 a-local carrier of substantially constant intensity Which is added to and in some cases entirely replaces the incoming fading carrier wave I.

In accordance with this further modification a portion of the amplified carrier signal output from I. F. carrier amplifier 3l is passed through lead 50 to second carrier-frequency oscillator 28, whereby the frequency of second oscillator 28 is locked at a frequency of 500 k. c. to the frequency of the incoming carrier signal present in I. F. amplifier 25.

Such carrier frequency energy is then com-l bined, with a minimum of undesirable distortion effects, with the incoming modulation sideband to reproduce the desired intelligence.

It is, therefore, clear that independently of the presence or absence of an incoming carrier wave, there is always available at detector 26 energy at carrier frequency which in the present example is the receiving intermediate frequency of 500 k. c.

Considering now in more detail the improved system of reception by which fading carrier distortion is reduced, it will be observed that a p0rtion of the output from I. F. amplifier 25 illustrated in Fig. 3, is selected by the I. F. band pass filter 2l and amplified in I. F. carrier amplifier 3l. A portion of the amplified fading carrier at a frequency of 500 k. c. is rectified in A. V. C. diode 42 and the A. V. C. bias thereby produced is used to control the gain of radio-frequency amplifier 22, I. F. amplifier 25 and I. F. carrier amplifier 31, the time constants of time constant circuits 43 and 44 being so arranged that undesirable violent fiuctuations in the level at detector 26 are avoided.

. In order, however, that exact synchronism be achieved in detector 2BV between the incoming carrier derived from I. F. amplifier 25 and the local carrier derived from carrier-frequency oscillator 28, additional synchronising means may be provided in the receiver.

Various methods have been suggested in the past for securing synchronism between a locally generated and an incoming carrier and any prior system may be employed without affecting the scope of the present invention.

synchronism is obtained between the local and incoming carriers, in the present example, by applying a further portion of the amplified output from I. F. carrier amplifier 3l at approximately 500 k. c. to frequency discriminator 38 through the lead 49 as illustrated in Fig. 3'.

Those skilled in the art are familiar with the construction and operation of frequency discriminator networks. They are commonly employed in connection with automatic frequency control (A. F. C.) systems used in conjunction with broadcast and like receivers. It is considered sufficient for the purpose of explaining this invention to state that in the output of the discriminator an A. F. C. potential is obtained, whose sign and magnitude vary in dependence upon and amount of the departure in frequency of the incoming carrier Wave signal from the mid-frequency of the discriminator. The output A. F. C. potential from discriminator 38 is applied through the controlled time constant device 39 both to first A. F. C. circuit 40 and also to second A. F. C. circuit 4|. First A. F. C. circuit 40 acts in a backward direction andv controls through lead 52 the frequency of mixeroscillator 24, while second A. F. C. circuit 4I is forward-acting and controls through lead 53 the frequency of second carrier-frequency oscillator 28.

It is not believed necessary to describe in detail the manner in which theA. F4, C. circuits 40 and 4l control the frequencies of oscillators24 and 28 respectively. In accordance with one well known system of A. F. C. thev circuits 40 and 4I may be constituted so as to reflect reactance across the tank circuits of their respective oscillators 24 and 28.

In the said known system frequency control tubes 40, 4l are connected to the tank circuits of the oscillators 24 and 28 in such a manner that variation of the gain of the control tube results in a change in effective reactance (capacitive or inductive) in a sense that will vary the frequency of the associated oscillator above or below a predetermined reference frequency.

The frequency control circuits may be of the type wherein the control tubes 40, 4l have their input capacity shunted across the tank circuit of theA oscillator whose frequency they control. A change in gain of the control tube, as for example, by variation of the bias potential applied to its control grid from the output of the discriminator, varies the effective capacity reactance of the associated tank circuits. On the 'other hand, the control circuits 40 and 4| may follow the teachings of Australian Patent No. 101,394. It is to be understood, however, that lthe frequency control network may be of any Awell known form. All that is essential to a proper understanding of this modification is that the operation of the frequency control circuits 4I and 4l should be such as will cause a variation in the tuning of the tank circuit of their associated oscillator in dependence upon the sign and magnitude of the control voltage applied to them from the discriminator 38.

'The backward-acting type of A. F. C. action of A. F. C. circuit 40 on mixer oscillator 24 is such as to compensate for undesired frequency Variation, whereby the frequency of the carrier signal in I. F. amplier 25 and I. F. carrier amplifier 31-.ap'proaches very closely to the mid-frequency of the discriminator 38, this mid-frequency being in the present example 500 k. c.

Forward-acting A. F. C. is provided through A. F. C. circuit 13| which controls the frequency of carrier-frequency oscillator 28, and the sense of this forward-acting A. F. C. is such as to displace the frequency of the carrier-frequency oscillator 28 from its uncontrolled frequency of 500 k. c. to a frequency higher than 500 k. c. when the controlled frequency of the incoming carrier in I. F. amplifier 25 is also higher than 500 k. c. Similarly in cases where backwardacting A. F. C. of mixer-oscillator 24 produces in I. F. amplifier 25 an incoming carrier-frequency slightly lower than 500 k. c., the forwardacting A. F. C. of carrier-frequency oscillator 28 displaces the frequency of the local carrier to a controlled frequency which is also slightly lower than 500 k. c.

It is, therefore, apparent that the combination of backward-acting A. F. C. of mixer-oscillator 2'4 with forward-acting A. F. C. of carrierfrequency oscillator 28 is eiiective in bringing i'nto close synchronism the frequencies of the incoming and the local carriers present in detector 2t).

It is important, however, yto consider A. F. C. actionv when a fading carrier is being received. Itis clear, inthe first place, that it is not feasible to lock the frequencies of the incoming and local carriers at such times as' the energy of the incoming carrier in I. F. carrier amplifier 31 is insuiiicient for this purpose. In the second place it is well known that A. F. C. action fails in discriminator such as 38 when no energy at carrier frequency is available in the output of I. F. carrier amplifier 3l, that is to say, when the fading carrier has substantially Zero intensity, the effects then being that the frequency of the mixer-oscillator 2li drifts back to its uncontrolled value and the frequency of the incoming carrier energy in I. F. amplier 25 drifts away correspondingly from the mid-frequency of the discriminator 38. At the same time the frequency of the carrier-frequency oscillator 28 tendsto return to its uncontrolled value and the net eli'ect is that synchronism no longer obtains between the incoming and local carrier injected into detector 26.

It is, therefore, desirable that means lbe provided for prolonging the A. F. C. action in the receiver in order that synchronism between the incoming and local carriers be maintained during periods of fading.

Various arrangements may be provided for achieving this effect, the nature of the arrangement being dependent on the type of A. F. C. systern employed in the circuits 40, 4l.

In accordance with a further modication of the receiver under discussion, the controlled time constant device, generally indicated at 39, is provided for this purpose and arranged to control the time constant of the A. F. C. Ibias which is supplied from the discriminator 3B to the control valves 40 and 4I. It consists of a time constant condenser (not shown) which is arranged so as to be effectively connected in shunt with the A. F. C. bias potential output fromdiscriminator 38 during periods of high carrier intensity. When, however, the intensity of the incoming carrier wave falls below the useful threshold value the time constant condenser (not shown) is effectively isolated from the discriminator 38 and thus holds its charge for a relatively long period. Consequently the potential difference existing across the condenser does not fall but remains at a value corresponding to the detuning of the receiver.

The desired control of the time constant of the A. F. C. bias circuit may be achieved by the use of biased diodes arranged in association with the discriminator 38 and the time constant condenser in such a manner that during periods of high carrier intensity they are conducting and pass the A. F. C. potential from the discriminator to the time constant condenser, while during periods of low carrier intensity they are non-conductive and the time constant condenser is effectively isolated from the discriminator.

The receiver for receiving intelligence, in accordance with this invention, may be still further modiiied as shown in Fig. 4. Referring to this figurea portion of the output from I. F. amplifier 25 is applied to I. F. carrier rejector 54 whose filter characteristic is such that carrier energy at a frequency of approximately 500 k. c. is greatly attenuated, whereas modulation sidebands in the range of frequencies from 500,250 cycles to 505,500 cycles are passed, substantially without attenuation, to rectifier 55. A portion of the output from rectifier 55 is selected by the 3 k. c. pass filter 55, whose lter characteristic is such that an audiofrequency tone of about 3 k. c. is passed substantially without attenuation, whereas signals at other frequencies are greatly attenuated, and the output from.filter 56 is amplified in 3 k. c. amplifier 51. A portion of the output from amplifier 51 at a frequency of 3 k. c. is conducted directly to first oscillator 33 whose frequency is thereby locked to that of the first oscillator 1 in the transmitting apparatus indicated by Fig. 2. A further portion of the 3 k. c. output from amplifier 5l is passed to a frequency discriminator 58 whose mid-frequency is, for example, 3 k. c. The differential A. F. C. bias potential developed by second discriminator 53 is applied through a controlled time constant device 59 to an A. F. C. valve 60 which automatically controls the frequency of first oscillator 33. The remainder of the apparatus indicated by Fig. 4 is similar to that bearing like numbers in preceding diagrams.

The method of operation of the alternative arrangement indicated by Fig. 4 is as follows:

The output from I. F. amplifier 25 comprises a carrier signal at a frequency of 500 k. c., a modulation sideband occupying a band of frequencies between 500,250 cycles and 502,500 cycles, and a duplicate modulation sideband occupying a band of frequencies between 503,250 cycles and 505,500 cycles. Considering, by way of example, a specified tone at a frequency of 400 cycles derived from the source of L. F. intelligence 4 indicated in Fig. 2, this tone is transmitted through I. F. amplifier 25 as a modulation signal at a frequency of 500,400 cycles and again as a duplicate modulation signal at a frequency of 503,400 cycles. I, F. carrier rejector 54 attenuates the carrier signal at a frequency of 500 k. c., but passes the modulation signals at frequencies of 500,400 cycles and 503,400 cycles respectively to rectifier 55 whose output, therefore, contains a combination tone at a difference frequency of 3 k. c. It is important at this stage to notice that the output from rectifier 55 contains a tone at a frequency of 3 k. c. independently of the precise frequency of the tone derived from the source of L. F. intelligence 4, since in all cases the intelligence is transmitted in duplicate modulation sidebands in which the frequencies of corresponding signals always differ precisely by a frequency of 3 k. c., as previously explained. The 3 k. c. tone is, therefore, selected by pass filter 56, amplified in 3 k. c. amplifier 51 and passed directly to first oscillator 33, whose frequency is thereby locked in known manner to that of first oscillator 1 in the transmitter.

In some cases, however, the energy available at a frequency of 3 k. c. in the output of amplifier 51 may be insufficient for the purpose of locking the frequencies of first oscillators 33 and 1 respectively. Such cases occur. for example, when selective fading prevents the reception of one or other of the duplicate modulation sidebands and it may then happen that. whereas the I. F. amplifier 25 transmits a modulation signal at a frequency of 500.400 cycles, the duplicate modulation signal at a frequency of 503.400 cycles is not available at appreciable intensity. The present modification, therefore. provides additional means for controlling the frequency of first oscillator 33,

utilizing the A. F. C. action of third A. F. C. circuit 60. The method of operation of the discriminator 58. the controlled time constant device 59 and the A. F. C. circuit 60 is in principle similar to that of the discriminator 38, the controlled time constant device 39 and forward-acting A. F. C. circuit 4|, although in the latter case the midfrequency of discriminator 38 is 500 k. c., whereas in the present case the mid-frequency of discriminator 58 is 3 k. c. Nevertheless. it is not believed to be necessary to describe in detail the method of operation of A. F. C. circuit 60. it being sufficient to point out that it is desirable to consider two sets of operating conditions corresponding respectively to periods duringr which the 3 k. c. output from amplifier 51 is either at high or low intensity. If. therefore. the frequency of first oscillator` 1 in the transmitter drifts away from the mid-frequency of the discriminator 58, an A. F. C. bias potential is produced in the output of discriminator 58 of sign and magnitude varying in dependence on the sense and amount of the departure in frequency of oscillator 1 from the mid-frequency of discriminator 58, and this A. F. C. bias potential is transferred through time constant device 59 to A. F. C. circuit 60. Thus variations in reactance of the A. F. C. circuit 60, corresponding with variations in applied A. F. C. potential, are utilized to control automatically the frequency of first oscillator 33, whereby the frequencies of first oscillators 1 and 33 are brought into close synchronism, as desired. When, however, the 3 k. c. output from amplifier 51 is at a low intensity, the A. F. C. potential developed by discriminator 58 is not applied to A. F. C. circuit 60, since the controlled and biased diodes included in controlled time constant device 59 are not conducting and thereby the A. F. C. circuit E0 is effectively isolated from the discriminator The reactance of the A. F. C. network and, therefore, the frequency of first oscillator 33, remain at values determined by the magnitude of the A. F. C. potential developed by discriminator 58 during the preceding period of high intensity of the 3 k. c. carrier output from amplifier 51. It follows that the oscillators 1 and 33 remain in close synchronism, independently of the effects of selective fading, since during periods of high intensity of output from 3 k. c. amplifier 51 the effect is that the frequency of first oscillator 33 is controlled by A. F. C. action of network 60, and furthermore the frequencies of first oscillators 1 and 33 are locked together, whereas during pe riods of low intensity of output from 3 k. c. amplifier 51 the effect then is that the discriminator 58 is isolated from the A. F. C. circuit 60 whose reactance then remains at the Value determined by the state of detuning of oscillator 33 existing during the preceding period of high intensity of output from 3 k. c. amplifier 51.

It is believed that the analogy between the methods of operation of the A. F. C. systems described in connection with Figs. 3 and 4 is sufficiently close to be readily understood by those skilled in the art.

Fig. 5 indicates a still further alternative in the receiving apparatus employed for the reception of intelligence in accordance with the invention and is to be read in conjunction with the preceding diagrams in which like parts bear like numbers.

A portion of the output from amplifier 51 is conducted to modulator 32 to which there is also supplied as previously mentioned the output from high-class filter 3| comprising duplicate modulation signals in the range of frequencies from 3,250 cycles to 5,500 cycles. The output from modulator 32 is led, as previously described, through second low pass filter 34 and through channel combining amplifier 35 to line or telephone output 35. A further portion of the output from 3 k. c. amplifier 51 is, however, conducted to anti-muting rectifier 6l whose output is passed to muting valve 63 through alternating current stopper circuit B2, whose filter characteristic is such that direct currents are passed substantially without attenuation whereas alternating currents are highly attenuated. Muting valve 53 is interposed between channel splitting amplifier 29 and first low pass filter 30 and is, therefore, in the chain of components through which passes audio-frequency intelligence from detector 26 to line output 36.

The method of operation of the apparatus indicated by Fig. 5 is as follows:

The output from I. F. amplifier 25 comprises a carrier signal at a frequency of 500 k, c., a modulation sideband occupying a band of frequencies between 500,250 cycles and 502,500 cycles, and a duplicate modulation sideband occupying a band of frequencies between 503,350 cycles and 505,500 cycles. Considering, by way of example, a specified tone at a frequency of 400 cycles derived from the source of L. F. intelligence 4 indicated in Fig. 2, this tone is transmitted through I. F. amplifier 25 as a modulation signal at a frequency of 500,400 cycles and again as a duplicate modulation signal at a frequency of 503,400 cycles. I. F. carrier rejector 54 attenuates the 500 k. c. carrier but passes the modulation signals at frequencies of 500,400 cycles and 503,400 cycles respectively to rectifier 55, whose output, therefore, contains a combination tone at the difference frequency of 3 k. c. 'Ihe 3 k. c, tone is then selected by pass filter 56 and amplified in 3 k. c. amplifier 51, a portion of whose output is supplied to modulator 32. The output from I. F. amplifier 25, together with the energy derived by conducting lead 48 from second carrier frequency oscillator 28 at a local carrier frequency of 500 k. c., is supplied to detector 26 whose output, containing signals at 400 cycles and 3,400 cycles, is conducted to channel splitting amplifier 29. The duplicate signal at 3,400 cycles is selected by high pass filter 3l and supplied to modulator 32 to which there is also supplied 3 k. c. energy derived from 3 k, c, amplifier 51. The output from modulator 32, therefore, contains the desired combination tone at the difference frequency of 400 cycles and this compensating energy is then selected by second low pass filter 34 and passed through channel combining amplifier 35 to line or telephone output 36.

It is important to notice that the energy contained in the duplicate modulation sideband at a frequency of 500,400 cycles has been converted, in the chain of components which includes modulator 32, into compensating energy at a frequency of 400 cycles in the output circuit 36. This result is achieved in the apparatus indicated by Fig. in spite of the fact that in the figure last mentioned the first oscillator 33 indicated in Fig. 3 has been eliminated. It is, however, clear that the conversion of duplicate energy at 3,400 cycles to compensating energy at 400 cycles is obtained in modulator 32 by virtue of the fact that the required demodulating energy at 3 k. c. is derived from 3 k. c. amplifier 51. The arrangement of apparatus indicated in Fig. 5, therefore, has the advantage that modulator 32 functions with a minimum of distortion, since the combination tone at difference frequency between the signals at 3,400 cycles and 3 k. c. is exactly 400 cycles as desired, independently of the effects of controlled variations in local oscillators in the receiver. This result follows since the frequency of the output from 3 k. c. amplifier 51 is to be traced back to the frequency of first oscillator 1 in the transmitting apparatus indicated by Fig. 2, whereas the frequency of the 3,400 cycle duplicate signal depends only on the frequencies respectively of the original 400 cycle tone derived from source 4 and of the first oscillator 1.

A still further advantage of the apparatus indicated by Fig. 5 is that the receiver is effectively silenced during the intervals between the reception of intelligence, e. g. speech, but is unblocked by voice-operated devices. Considering, firstly the chain of components including modulator 32 through which the duplicate energy at 3,400 cycles is transmitted, it is clear that there is substantially no energy in the output from modulator 32 at a frequency which is within the pass-band of second low pass filter 34 during periods when the 3 k, c. output from amplifier 51 is negligibly small. Since, moreover, there is appreciably no signal energy in 3 k. c. amplifier 51, unless modulation sidebands are being received, it is clear that modulator 32 is effectively silenced at all times except when intelligence is being transmitted in the tele-communication system. Turning now to the chain of components including first low pass lter 30 through which the signal energy at 400 cycles is transmitted, it is observed that muting valve 63 is effectively silenced during periods when no anti-muting bias voltage is available in the output of anti-muting rectifier 6l. Since, moreover, anti-muting rectifier 6I is fed from the output of 3 k. c. amplifier 51, it is clear that muting valve 63 is effectively silenced at all times except when intelligence is being transmitted as modulation sidebands in the telecommunication system. It is not believed necessary to describe in detail the method of operation of the chain of components including anti-muting rectifier 6l, alternating current stopper 62 and muting valve 63, it being sufficient to point out that muting valve 63 may, for example, include a signal grid to which the output from channel splitting amplifier 29 is applied and a control grid to which are applied in opposition a steady negative muting bias voltage of value sufficient to cut-off anode-cathode current, and a uni-"directional positive anti-muting bias voltage ofmagnitude depending on the intensity of the 3 k. c. signal input to anti-muting rectifier 6i.

Although the invention has been described particularly in relation to the tele-communication system indicated by the accompanying diagrams, it is clear that a number of modifications of the system are available without departing from the spirit of the invention. Nevertheless, it must particularly be pointed out that the telecommunication system disclosed by the diagrams is well adapted to long-distance radiotelephone speech circuits and that the system as a. whole represents optimum conditions with respect to economy of transmitted power, reduction of frequency bandwidth required for the transmitted waves, ease of recombination in the receiver of the transmitted waves in order to reproduce the original intelligence, and more generally with respect to an economic balance of cost and complexity between transmitter and receiver. This result is achieved according to the invention by the combination of apparatus provided at the transmitter for the radiation of duplicate H. F. modulation side-bands whereby the undesirable defects of selective fading are overcome, and of apparatus provided at the receiver for the production of a local carrier signal whereby the undesirable defects of carrier fading are overcome.

Reverting now to the tele-communication system indicated by Fig. 1, it is apparent that the duplicate H. F. modulation side-bands may, if desired, be radiated in association with a plurality of carrier wave signals disposed on adjacent frequencies, as described in Australian Patent No. 103,640, whereby the defects of fading carrier distortion are markedly reduced. In

such systems, however, it is desirable that dis- Yband and a duplicate upper H. F. modulation sideband. Clearly, however, there is no departure from the spirit of the invention if lower sidebands are radiated rather than the upper sidebands illustrated in Fig. l. By way of example, the lower sidebands may occupy a band of frequencies from 9,999,750 cycles to 9,997,500 cycles in which the former limiting frequency corresponds to a L. F. of 250 cycles in the intelligence. Similarly the duplicate lower sideband has limiting frequencies of 9,996,750 cycles and 9,994,500 cycles of which the former corresponds with a 250 cycle tone in the intelligence, whereas the 9,994,500 cycle frequency corresponds with the 2,500 cycle tone. The corresponding changes in the transmitting apparatus indicated by Fig. 2 may readily be visualized. For example, third band pass filter I4 passes, substantially without attenuation, signals having frequencies between 100 k. c. and 94,500 cycles, that is to say, a second carrier at a frequency of 100 k. c., a medium frequency lower modulation sideband having frequencies within the range from 99,750 cycles tc 97,500 cycles and a duplicate medium frequency lower modulation sideband having frequencies within the range from 95,750 cycles to 94,500 cycles. In these medium frequency sidebands it is apparent that the limiting frequencies 99,750 cycles and 96,750 cycles both correspond with a 250 cycle tone in the intelligence derived from source 4, whereas limiting frequencies 97,500 cycles and 94,500 cycles both correspond with a 2,500 cycle tone. Similarly fourth band pass filter I8 passes to aerial 20 a carrier signal at a frequency of 10 m. c. and lower H. F. modulation sidebands occupying the band of frequencies between 9,999,750 cycles and 9,994,500 cycles.

Turning again to the transmitting apparatus indicated by Fig. 2, it has already been mentioned that the carrier signal at a frequency of 10 m. c. may be radiated with reduced intensity as a socalled pilot carrier. On the other hand, it is not essential that the signals in duplicate H. F. modulation sideband 3 occupying the band of frequencies between 10,003,250 cycles and 10,005,500 cycles be radiated with intensities equal to those of the corresponding signals in H. F. modulation sideband 2 extending from 10,000,250 cycles to 10,002,500 cycles. In some cases where the eiiects of selective fading are less severe it is feasible to interpose between second amplifier 9 and channel combining amplifier Il an attenuator which functions to reduce the intensities of the duplicate signals within the frequency band 3,250 cycles to 5,500 cycles relative to the intensities of the speech signals in the output of first amplifier l0.

I claim:

1. In an intelligence transmitting and receiving system, means for obtaining low frequency potentials corresponding to the desired intelligence, means for utilizing a portion of said low frequency potentials in combination with an oscillation of a fixed frequency to produce a band of frequencies corresponding to the sum of said fixed frequency and the frequencies of said low frequency potentials, means for selecting said band of sum frequencies, means for amplifying said low frequency and said band of sum frequencies, means for modulating a common high frequency carrier with said low frequency and with said band of sum frequencies, means for selecting the carrier and sidebands adjacent on one side of said carrier, means for radiating said selected carrier and adjacent sidebands, and means for receiving and utilizing the radiation so obtained.

2. A radio transmitting and receiving system suitable for reduction of fading effects, comprising transmitting equipment having a low frequency intelligence source, a principal carrier frequency source, a plurality of sub-carrier frequency sources, means for modulating one of said sub-carrier frequency sources by said intelligence source thereby to produce different sidebands, band-pass filter means to pass one only of said sidebands, means for combining the filter-passed sideband with amplified energy from said intelligence source, modulation means receptive of energy from said combining means and from a second of said sub-carrier frequency sources, means for filter-passing two wanted sidebands from the output of the last said modulation means, means for modulating energy from said principal carrier frequency source by sidebands derived from the last-mentioned filter-passing means thereby to produce at least four separated sidebands, a final filter means for passing a selected pair of said sidebands together with their principal carrier, means for radiating the output from said final lter means, and means for receiving and utilizing the radiation so obtained.

3. A system in accordance with claim 2 and including means for maintaining a frequency separation between the sidebands resulting from the modulation of said sub-carriers which is independent of the frequency of said intelligence source.

4. A system in accordance with claim 2 and including means for suppressing those of the modulation sidebands whose frequencies lie between the frequencies of said sub-carriers.

5. A system for overcoming fading effects in radio communication comprising means for producing a selected band of sum frequencies which result from the simultaneous modulation of a sub-carrier wave by a variable low frequency wave and by a sum-frequency mixture of said variable low frequency wave with a constant low frequency, means for transmitting at least one carrier and a pair of separated 'side bands located on the same side of said carrier and having similar modulation components which are constituted as derivatives from the first said means.

6. 'I'he method of overcoming fading in a radio system which includes modulating a fixed low frequency wave by energy from a source of 10W frequency intelligence, filter-passing the sum' frequencies so obtained, combining said sum frequencies with amplified energy from said source, modulating a sub-carrier wave by the combined intelligence frequencies and sum frequencies to obtain two separated sidebands on each side of said sub-carrier wave, filtering the last said modulation products to suppress the two sidebands which lie below the sub-carrier, modulating a high frequency carrier wave by the remaining filter products, band-pass filtering said high frequency carrier wave and the two separated sidebands which lie above the frequency of the carrier wave, radiating said carrier wave and its two upper sidebands so derived, receiving the energy so radiated, and reducing the same to intelligence.

ALFRED LEONARD GREEN. 

